With increasing development of science and technology, portable electronic devices such as smart phones, tablet personal computers or MP3 players are now rapidly gaining in popularity. Generally, these portable electronic devices are powered by a fixed voltage (e.g. 5V). Each of the portable electronic devices is usually equipped with at least one first connecting port (e.g. USB port). Via the first connecting port, the portable electronic device is connected with a power supply system to receive a charging voltage.
Generally, the power supply system comprises a power adapter, a power cable, and a USB hub. The USB hub comprises plural second connecting ports (e.g. USB ports). The plural second connecting ports of the USB hub may be connected with the first connecting ports of corresponding portable electronic devices. The power adapter is used for receiving an input voltage (e.g. an AC voltage from a utility power source) and converting the input voltage into an output voltage (e.g. 5V) for powering the portable electronic devices. The power cable is connected between the power adapter and the USB hub. Through the power cable, the output voltage from the power adapter may be transmitted to the USB hub. After the first connecting ports of the portable electronic devices are connected with the second connecting ports of the USB hub, the electric energy of the output voltage may be transmitted to the portable electronic devices to charge the portable electronic devices.
As known, the length of the power cable is related to a corresponding impedance value of the power cable. As the length of the power cable is increased, the impedance value is increased. Since the level of the output voltage from the power adapter is fixed, the output voltage is subjected to a voltage drop. Due to the voltage drop, the voltage level at the USB hub is smaller than the level of the output voltage from the power adapter. Under this circumstance, the voltage level provided by the USB hub possibly fails to reach the desired voltage level for powering the portable electronic devices. Moreover, as the number of the portable electronic devices connected with the USB hub is increased, the magnitude of a load current outputted from the power adapter is increased, and the voltage drop across the power cable is increased. Under this circumstance, the voltage level at the USB hub is further decreased. Due to the impedance of the power cable, the voltage level provided by the USB hub possibly fails to reach the desired voltage level for powering the portable electronic devices. If the voltage level provided by the USB hub is insufficient, the chargeable batteries of the portable electronic devices fail to be fully charged and thus the aging conditions of the chargeable batteries are accelerated. Under this circumstance, the endurance of the chargeable batteries will be impaired.
Therefore, there is a need of providing an improved power adapter for dynamically adjusting an output voltage in order to obviate the above drawbacks. Consequently, the level of the output voltage from the power adapter is correspondingly adjusted according to the voltage drop resulting from the impedance of the power cable.